The present invention relates to novel polyethylene compositions.
Many different grades of polyethylene are manufactured for different applications, and equally there is a wide variety of physical properties of polyethylene which are important in each case. Generally it is not just one property which is important for a particular application, but several: finding a polyethylene which possesses the right combination of those properties is the major objective of much research. For example, in pipe and moulding applications properties such as density, viscosity (i.e. melt flow rate), impact strength and rigidity are all important.
The use of certain transition metal compounds to polymerise 1-olefins, for example, ethylene, is well established in the prior art. Silica-supported chromium catalysts using the Phillips process have been known for several decades. The use of Ziegler-Natta catalysts, for example those catalysts produced by activating titanium halides with organometallic compounds such as triethylaluminium, is fundamental to many commercial processes for manufacturing polyolefins. In recent years the use of certain metallocene catalysts (for example biscyclopentadienylzirconiumdichloride activated with alumoxane) has provided catalysts with potentially high activity. These different catalyst systems provide polymeric products with a variety of properties.
Commodity polyethylenes are produced commercially in a variety of different types and grades. Homopolymerisation of ethylene with transition metal based catalysts leads to the production of so-called xe2x80x9chigh densityxe2x80x9d grades of polyethylene. These polymers have relatively high stiffness and are useful for making articles where inherent rigidity is required, such as pipe and moulded products.
WO98/27124, published after the earliest priority date of this invention, discloses that ethylene may be polymerized by contacting it with certain iron or cobalt complexes of selected 2,6-pyridinecarboxaldehydebis(imines) and 2,6-diacylpyridinebis(imines). There is no disclosure regarding the properties of polyethylene produced by such catalysts and because the polyethylene produced in those Examples where the molecular weight is higher than oligomeric mostly shows extremely broad molecular weight distributions, it would not have the range of properties considered in this application.
We have discovered a class of novel homopolymers of ethylene which have a combination of properties that make them particularly suitable for use in pipe, film and moulded products. Accordingly a first aspect of the invention provides a homopolymer of ethylene which has:
an annealed density D/weight average molecular weight MW relationship defined by the equation equation D greater than 1104.5MWxe2x88x920.0116; and
either a Charpy Impact I/High Load Melt Index H relationship defined by the equation I greater than 35.0Hxe2x88x920.4,
or a dynamic storage modulus Gxe2x80x2 of 2.9 or less.
Weight average molecular weight MW is measured by GPC. Annealed density is measured to specification ISO 1872-1:1993 using test method ISO 1183:1987. Charpy impact is measured according to ISO 179-1982/2/A on sheets compression moulded according to specification BS EN ISO 1872-2:1997. High Load Melt Index (HLMI) is a commonly used measure, which like MFR gives an indication of melt viscosity and hence molecular weight. It is determined by a melt indexer in terms of the melt output (g/10 minutes) under a given high load (21.6 kg) through a standard die orifice. In this application HLMI is measured according to ASTM D 1238 condition F, 21.6 kg at 190xc2x0 C.
Dynamic storage modulus Gxe2x80x2 is formally defined as the storage modulus measured at a loss modulus (Gxe2x80x3) of 5 kPa. It is essentially the modulus of the melt measured xe2x80x9cin phasexe2x80x9d with the imposed oscillation in a dynamic test, and can be considered to quantify the elasticity of the melt. The steady state compliance (Jso) is a viscoelastic property of polymers. Methods for measuring Jso, and a discussion of its utility, can be found in a number of text books (see for example Chapters 2 and 10 of xe2x80x9cMelt Rheology and its Role in Plastics Processing, Theory and Applicationsxe2x80x9d, by John M. Dealy and Kurt F. Wissbrun, published by Van Nostrand Reinhold, New York, 1990). Jso is recognised as a useful property for polymer characterisation, and has been found to be independent of a polymer""s average molecular weight but strongly affected by its molecular weight distribution, particularly by the fraction of very high molecular weight polymer present. Measurement of Jso or some associated melt viscoelastic property is a far more sensitive method for characterising polymers for subtle differences in molecular weight distribution than are dilute solution measurements. However Jso is difficult to measure directly for high molecular weight polyethylenes, and therefore an indirect method is used: it can be related to the storage modulus. (Gxe2x80x2), measured in a dynamic test at low frequency (xcfx89), by the relationship
Gxe2x80x2(xcfx89)=Jso[Gxe2x80x3(xcfx89)]2 for xcfx89xe2x86x920
where Gxe2x80x3 is the loss modulus, also measured at low frequency. In practice therefore, it is possible to measure Gxe2x80x2 at a low reference value of Gxe2x80x3, and to use this parameter as an indication of the fraction of very high molecular weight polymer present. The method for measuring Gxe2x80x2 is described in the Examples below.
In a second aspect the invention provides a homopolymer of ethylene which has a polydispersity MW/Mn of 16 or less, and and wherein the width of its molecular weight distribution at half the peak height is at least 1.6. The width of the molecular weight distribution is measured on a logarithmic scale.
Preferably the polydispersity MW/Wn is between 7 and 16. Number average molecular weight Mn like MW is measured by GPC according to NAMAS method MT/GPC/02. At such relatively low polydispersities we have found that the homopolymers of the invention have a distinctive molecular weight distribution which can be expressed mathematically in the above manner. It is believed that this may at least partly account for some of the novel properties recited below.
Preferably the annealed density/molecular weight relationship is defined by the equation D greater than 1105.5MWxe2x88x920.0116. The Charpy Impact/HLMI relationship is preferably defined by the equation I greater than 37.0Hxe2x88x920.42, more preferably I greater than 38.8Hxe2x88x920.42.
Whilst polyethylene homopolymers are known which have properties defined by at least one of the above relationships, none has properties defined by both the density and Charpy Impact relationships. This unique combination of properties makes the polyethylene of the invention particularly suitable for a number of applications. For example, the improved density: melt mass-flow rate (MFR) performance (MFR being inversely proportional to molecular weight) of the compounds of the invention means that for a given MS it is possible to produce articles such as bottles or drums with a higher rigidity: weight ratio. This is particularly advantageous for the production of fast-cycling thin-walled bottles. The higher impact strength: MFR ratio is advantageous for drum or large container applications either to improve the impact strength of a container for a given weight, or to reduce the weight for a given impact strength. Thus the compounds of the invention enable containers to be made with reduced weight whilst maintaining both rigidity and impact strength.
It is also preferred that the homopolymer has an MFR drop on compounding of 20% or less when the HLMI is less than 10. By xe2x80x9cMFR drop on compoundingxe2x80x9d is meant the difference between the Melt Flow Ratio of compounded pellets of the homopolymer and the MFR of the powder before compounding. The MFR of polyethylene generally drops upon compounding: the smaller the drop, the smaller the change in processing and viscosity properties upon compounding from powder into pellets. Thus the relatively small MFR drop experienced by the homopolymers of the invention upon compounding is advantageous as it indicates that the they can be compounded with minimal changes in properties. Melt mass-flow rate of the polymers is measured to ISO 1133:1997xe2x80x94Part 7. The values quoted for MFR in this specification are in dg/min.
Preferably the homopolymer also has a relationship of die swell S (at shear rate 15/s and 190xc2x0 C.) to HLMI H defined by the equation S less than 10log10H+30, preferably S less than 10log10H+29, and more preferably S less than 10log10H+28. It is preferred also that the homopolymer has a polydispersity (MW/Mn) of less than 30.
Preferred homopolymers also contain vinyl end-groups. Generally the vinyl content is greater than 0.3 per 1000 carbons (0.3/1000C), and preferably greater than 0.5/1000C. The number of vinyls per 1000C is determined by pressing a film of the polymer at 150xc2x0 C., and obtaining a spectrum at 2 cmxe2x88x921 resolution. The vinyl concentration is determined from the 909 cmxe2x88x921 waveband according to the formula
vinyl content/1000C=(14. A909)/d.t.E
where
A909=absorption at 909 cmxe2x88x921, d=density, t=film thickness, E=molar absorptivity of vinyl group.
A further aspect of the invention provides a film of a polymer of ethylene, which film has
a density of at least 957 kg/m3;
a Dart Impact of at least 130 g; and
a polydispersity of less than 12.
The term xe2x80x9cfilmxe2x80x9d in this context means a blown film having a thickness of 100 xcexcm or less. The film is preferably a homopolymer of ethylene. Dart Impact is a well-known test in the art, and effectively gives a measure of the force required to push a hole through a taut film. Details of the test are given in the Examples below. Preferably the Dart Impact is at least 140 g, and more preferably at least 150 g.
The polymers of the invention may be conveniently produced using a polymerisation catalyst comprising a compound of the Formula B: 
wherein M is Fe[II], Fe[III], Co[I], Co[II], Co[III], Mn[I], Mn[II], Mn[III], Mn[IV], Ru[II], Ru[III] or Ru[IV]; X represents an atom or group covalently or ionically bonded to the transition metal M; T is the oxidation state of the transition metal M and b is the valency of the atom or group X; R1, R2, R3, R4, R5, R6 and R7 are independently selected from hydrogen, halogen, hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl; and when any two or more of R1-R7 are hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl, said two or more can be linked to form one or more cyclic substituents.
The atom or group represented by X in the compound of Formula B are preferably selected from halide, sulphate, nitrate, thiolate, thiocarboxylate, BF4xe2x88x92, PF6xe2x88x92, hydride, hydrocarbyloxide, carboxylate, hydrocarbyl, substituted hydrocarbyl and heterohydrocarbyl. Examples of such atoms or groups are chloride, bromide, methyl, ethyl, propyl, butyl, octyl, decyl, phenyl, benzyl, methoxide, ethoxide, isopropoxide, tosylate, triflate, formate, acetate, phenoxide and benzoate.
It is preferred that in addition to (1) the compound of Formula B, the catalyst additionally incorporates (2) an activating quantity of an activator compound, preferably an organoaluminium compound or a hydrocarbylboron compound. Suitable organoaluminium compounds include trialkyaluminium compounds, for example, trimethylaluminium, triethylaluminium, tributylaluminium, tri-n-octylaluminium, ethylaluminium dichloride, diethylaluminium chloride and alumoxanes. Alumoxanes are well known in the art as typically the oligomeric compounds which can be prepared by the controlled addition of water to an alkylaluminium compound, for example trimethylaluminium. Such compounds can be linear, cyclic or mixtures thereof. Commercially available alumoxanes are generally believed to be mixtures of linear and cyclic compounds. The cyclic alumoxanes can be represented by the formula [R16AlO]s and the linear alumoxanes by the formula R17(R18AlO)s wherein s is a number from about 2 to 50, and wherein R16, R17, and R18 represent hydrocarbyl groups, preferably C1 to C6 alkyl groups, for example methyl, ethyl or butyl groups.
Examples of suitable hydrocarbylboron compounds are dimethylphenylammoniumtetra(phenyl)borate, trityltetra(phenyl)borate, triphenylboron, dimethylphenylammonium tetra(pentafluorophenyl)borate, sodium tetrakis[(bis-3,5-trifluoromethyl)phenyl]borate, H+(OEt2) [(bis-3,5-trifluoromethyl)phenyl]borate, trityltetra(pentafluorophenyl)borate and tris(pentafluorophenyl) boron.
In the preparation of the preferred catalysts for making the polymers of the present invention, the quantity of activating compound selected from organoaluminium compounds and hydrocarbylboron compounds to be employed is easily determined by simple testing, for example, by the preparation of small test samples which can be used to polymerise small quantities of the monomer(s) and thus to determine the activity of the produced catalyst. It is generally found that the quantity employed is sufficient to provide 0.1 to 20,000 atoms, preferably 1 to 2000 atoms of aluminium or boron per Fe, Co, Mn or Ru metal atom in the compound of Formula A.
The preferred polymerisation catalyst for use in the present invention preferably additionally comprises (3) a neutral Lewis base. Neutral Lewis bases are well known in the art of Ziegler-Natta catalyst polymerisation technology. Examples of classes of neutral Lewis bases suitably employed in the present invention are unsaturated hydrocarbons, for example, alkenes or alkynes, primary, secondary and tertiary amines, amides, phosphoramides, phosphines, phosphites, ethers, thioethers, nitriles, carbonyl compounds, for example, esters, ketones, aldehydes, carbon monoxide and carbon dioxide, sulphoxides, sulphones and boroxines. Although 1-olefins are capable of acting as neutral Lewis bases, for the purposes of the present invention they are regarded as monomer or comonomer 1-olefins and not as neutral Lewis bases per se. However, alkenes which are internal olefins, for example, 2-butene and cyclohexene are regarded as neutral Lewis bases in the present invention. Preferred Lewis bases are tertiary amines and aromatic esters, for example, dimethylaniline, diethylaniline, tributylamine, ethylbenzoate and benzylbenzoate. In this particular aspect of the present invention, components (1), (2) and (3) of the catalyst system can be brought together simultaneously or in any desired order. However, if components (2) and (3) are compounds which interact together strongly, for example, form a stable compound together, it is preferred to bring together either components (1) and (2) or components (1) and (3) in an initial step before introducing the final defined component. Preferably components (1) and (3) are contacted together before component (2) is introduced. The quantities of components (1) and (2) employed in the preparation of this catalyst system are suitably as described above in relation to the catalysts of the present invention. The quantity of the neutral Lewis Base [component (3)] is preferably such as to provide a ratio of component (1):component (3) in the range 100:1 to 1:1000, most preferably in the range 1:1 to 1:20. Components (1), (2) and (3) of the catalyst system can brought together, for example, as the neat materials, as a suspension or solution of the materials in a suitable diluent or solvent (for example a liquid hydrocarbon), or, if at least one of the components is volatile, by utilising the vapour of that component. The components can be brought together at any desired temperature. Mixing the components together at room temperature is generally satisfactory. Heating to higher temperatures e.g. up to 120xc2x0 C. can be carried out if desired, e.g. to achieve better mixing of the components. It is preferred to carry out the bringing together of components (1), (2) and (3) in an inert atmosphere (e.g. dry nitrogen) or in vacuo. If it is desired to use the catalyst on a support material (see below), this can be achieved, for example, by preforming the catalyst system comprising components (1), (2) and (3) and impregnating the support material preferably with a solution thereon or by introducing to the support material one or more of the components simultaneously or sequentially. If desired the support material itself can have the properties of a neutral Lewis base and can be employed as, or in place of, component (3). An example of a support material having neutral Lewis base properties is poly(aminostyrene) or a copolymer of styrene and aminostyrene (ie vinylaniline).
The following are examples of nitrogen-containing transition metal complexes (1) which may be used as catalysts to make the polymers of the invention:
2,6-diacetylpyridinebis(2,6-diisopropylanil)FeCl2 
2,6-diacetylpyridine(2,6-diisopropylanil)MnCl2 
2,6-diacetylpyridine(2,6-diisopropylanil)CoCl2 
2,6-diacetylpyridinebis(2-tert.-butylanil)FeCl2 
2,6-diacetylpyridinebis(2,3-dimethylanil)FeCl2 
2,6-diacetylpyridinebis(2-methylanil)FeCl2 
2,6-diacetylpyridinebis(2,4-dimethylanil)FeCl2 
2,6-diacetylpyridinebis(2,4,6-trimethylanil)FeCl2 
2,6-diacetylpyridinebis(2,6-dimethylanil)FeCl2 
2,6-dialdiminepyridinebis(2,6-dimethylanil)FeCl2 
2,6-dialdiminepyridinebis(2,6-diethylanil)FeCl2 
2,6-dialdiminepyridinebis(2,6-diisopropylanil)FeCl2 
2,6-dialdiminepyridinebis(1-naphthil)FeCl2 and
2,6-bis(1,1-diphenylhydrazone)pyridine.FeCl2.
The catalysts may contain a mixture of compounds such as, for example, a mixture of 2,6-diacetylpyridinebis(2,6-diisopropylanil)FeCl2 complex and 2,6-diacetylpyridinebis(2,4,6-trimethylanil)FeCl2 complex, or a mixture of 2,6-diacetylpyridine(2,6-diisopropylanil)CoCl2 and 2,6-diacetylpyridinebis(2,4,6-trimethylanil)FeCl2. In addition to said one or more defined transition metal compounds, the catalysts of the present invention can also include one or more other types of transition metal compounds or catalysts, for example, transition metal compounds of the type used in conventional Ziegler-Natta catalyst systems, metallocene-based catalysts, or heat activated supported chromium oxide catalysts (eg Phillips-type catalyst).
The catalysts of the present invention can be unsupported or supported on a support material, for example, silica, alumina, or zirconia, or on a polymer or prepolymer, for example polyethylene, polystyrene, or poly(aminostyrene).
If desired the catalysts can be formed in situ in the presence of the support material, or the support material can be pre-impregnated or premixed, simultaneously or sequentially, with one or more of the catalyst components. The catalysts of the present invention can if desired be supported on a heterogeneous catalyst, for example, a magnesium halide supported Ziegler Natta catalyst, a Phillips type (chromium oxide) supported catalyst or a supported metallocene catalyst. Formation of the supported catalyst can be achieved for example by treating the transition metal compounds of the present invention with alumoxane in a suitable inert diluent, for example a volatile hydrocarbon, slurrying a particulate support material with the product and evaporating the volatile diluent. The produced supported catalyst is preferably in the form of a free-flowing powder. The quantity of support material employed can vary widely, for example from 100,000 to 1 grams per gram of metal present in the transition metal compound.
The present invention further provides a process for producing homopolymer of ethylene which has:
an annealed density D/weight average molecular weight MW relationship defined by the equation equation D greater than 1104.5MWxe2x88x920.0116 
and a Charpy Impact I/High Load Melt Index H relationship defined by the equation I greater than 35.0Hxe2x88x920.4 
which process comprises contacting the ethylene under polymerisation conditions with a polymerisation catalyst comprising (1) a compound having the Formula B as defined above and optionally (2) an activating quantity of an activator compound comprising a Lewis acid capable of activating the catalyst for olefin polymerisation.
The invention also encompasses a homopolymer of ethylene having
an annealed density D/molecular weight M relationship defined by the equation equation D greater than 1104.5Mxe2x88x920.0116 
and a Charpy Impact I/High Load Melt Index H relationship defined by the equation I greater than 35.0Hxe2x88x920.4 
which polymer is obtainable by the above process.
A further aspect of the invention provides a process for making a film of a polymer of ethylene, which process comprises forming a polymer of ethylene which has:
an annealed density D/weight average molecular weight MW relationship defined by the equation equation D greater than 1104.5MWxe2x88x920.016 
and a Charpy Impact I/High Load Melt Index H relationship defined by the equation I greater than 35.0Hxe2x88x920.4 
by a process comprising contacting the ethylene under polymerisation conditions with a polymerisation catalyst comprising (1) a compound having the B as defined above and optionally (2) an activating quantity of an activator compound comprising a Lewis acid capable of activating the catalyst for olefin polymerisation, and then blowing the resultant polymer into a film.
In a preferred process, the film comprises an ethylene homopolymer.
The compound (1) and optionally activator (2) may be contacted with the olefin to be polymerised in the form of a single catalyst system, or they may be added to the reactor separately.
The polymerisation conditions employed in the process of the invention can be, for example, solution phase, slurry phase or gas phase. If desired, the catalyst can be used to polymerise the olefin under high pressure/high temperature process conditions wherein the polymeric material forms as a melt in supercritical ethylene. Preferably the polymerisation is conducted under gas phase fluidised bed conditions. Suitable monomers for use in the polymerisation process of the present invention are, for example, ethylene, propylene, butene, hexene, methyl methacrylate, methyl acrylate, butyl acrylate, acrylonitrile, vinyl acetate, and styrene. Preferred monomers for homopolymerisation processes are ethylene and propylene.
Slurry phase polymerisation conditions or gas phase polymerisation conditions are particularly useful for the production of high density grades of polyethylene. In these processes the polymerisation conditions can be batch, continuous or semi-continuous. In the slurry phase process and the gas phase process, the catalyst is generally fed to the polymerisation zone in the form of a particulate solid. In the case of compound (1), this solid may be an undiluted solid catalyst system formed from a nitrogen-containing complex and an activator, or can be the solid complex alone. In the latter situation, the activator can be fed to the polymerisation zone, for example as a solution, separately from or together with the solid complex. Preferably the catalyst system or the transition metal complex component of the catalyst system employed in the slurry polymerisation and gas phase polymerisation is supported on a support material. Most preferably the catalyst system is supported on a support material prior to its introduction into the polymerisation zone. Suitable support materials are, for example, silica, alumina, zirconia, talc, kieselguhr, or magnesia Impregnation of the support material can be carried out by conventional techniques, for example, by forming a solution or suspension of the catalyst components in a suitable diluent or solvent, and slurrying the support material therewith. The support material thus impregnated with catalyst can then be separated from the diluent for example, by filtration or evaporation techniques.
In the slurry phase polymerisation process the solid particles of catalyst, or supported catalyst, are fed to a polymerisation zone either as dry powder or as a slurry in the polymerisation diluent. Preferably the particles are fed to a polymerisation zone as a suspension in the polymerisation diluent. The polymerisation zone can be, for example, an autoclave or similar reaction vessel, or a continuous loop reactor, e.g. of the type well-know in the manufacture of polyethylene by the Phillips Process. When the polymerisation process of the present invention is carried out under slurry conditions the polymerisation is preferably carried out at a temperature above 0xc2x0 C., most preferably above 15xc2x0 C. The polymerisation temperature is preferably maintained below the temperature at which the polymer commences to soften or sinter in the presence of the polymerisation diluent. If the temperature is allowed to go above the latter temperature, fouling of the reactor can occur. Adjustment of the polymerisation within these defined temperature ranges can provide a useful means of controlling the average molecular weight of the produced polymer. A further useful means of controlling the molecular weight is to conduct the polymerisation in the presence of hydrogen gas which acts as chain transfer agent. Generally, the higher the concentration of hydrogen employed, the lower the average molecular weight of the produced polymer.
The use of hydrogen gas as a means of controlling the average molecular weight of the polymer or copolymer applies generally to the polymerisation process of the present invention. For example, hydrogen can be used to reduce the average molecular weight of polymers or copolymers prepared using gas phase, slurry phase or solution phase polymerisation conditions. The quantity of hydrogen gas to be employed to give the desired average molecular weight can be determined by simple xe2x80x9ctrial and errorxe2x80x9d polymerisation tests.
The preferred polymerisation process of the present invention provides polymers at remarkably high productivity (based on the amount of polymer produced per unit weight of nitrogen-containing transition metal complex employed in the catalyst system). This means that relatively very small quantities of catalyst are consumed in commercial processes using the process of the present invention. It also means that when the process of the present invention is operated under polymer recovery conditions that do not employ a catalyst separation step, thus leaving the catalyst, or residues thereof, in the polymer (e.g. as occurs in most commercial slurry and gas phase polymerisation processes), the amount of catalyst in the produced polymer can be very small. Experiments carried out with the preferred transition metal catalyst utilised in the present invention show that, for example, polymerisation of ethylene under slurry polymerisation conditions can provide a particulate polyethylene product containing catalyst so diluted by the produced polyethylene that the concentration of transition metal therein falls to, for example, 1 ppm or less wherein xe2x80x9cppmxe2x80x9d is defined as parts by weight of transition metal per million parts by weight of polymer. Thus a preferred polyethylene homopolymer or polyethylene film according to the present invention has a transition metal content of, for example, in the range of 1-0.0001 ppm, preferably 1-0.001 ppm.
Methods for operating gas phase polymerisation processes are well known in the art. Such methods generally involve agitating (e.g. by stirring, vibrating or fluidising) a bed of catalyst, or a bed of the target polymer (i.e. polymer having the same or similar physical properties to that which it is desired to make in the polymerisation process) containing a catalyst, and feeding thereto a stream of monomer at least partially in the gaseous phase, under conditions such that at least part of the monomer polymerises in contact with the catalyst in the bed. The bed is generally cooled by the addition of cool gas (e.g. recycled gaseous monomer) and/or volatile liquid (e.g. a volatile inert hydrocarbon, or gaseous monomer which has been condensed to form a liquid). The polymer produced in, and isolated from, gas phase processes forms directly a solid in the polymerisation zone and is free from, or substantially free from liquid. As is well known to those skilled in the art, if any liquid is allowed to enter the polymerisation zone of a gas phase polymerisation process the quantity of liquid is small in relation to the quantity of polymer present in the polymerisation zone. This is in contrast to xe2x80x9csolution phasexe2x80x9d processes wherein the polymer is formed dissolved in a solvent, and xe2x80x9cslurry phasexe2x80x9d processes wherein the polymer forms as a suspension in a liquid diluent.
The gas phase process can be operated under batch, semi-batch, or so-called xe2x80x9ccontinuousxe2x80x9d conditions. It is preferred to operate under conditions such that monomer is continuously recycled to an agitated polymerisation zone containing polymerisation catalyst, make-up monomer being provided to replace polymerised monomer, and continuously or intermittently withdrawing produced polymer from the polymerisation zone at a rate comparable to the rate of formation of the polymer, fresh catalyst being added to the polymerisation zone to replace the catalyst withdrawn form the polymerisation zone with the produced polymer.
In the preferred embodiment of the gas phase polymerisation process of the present invention, the gas phase polymerisation conditions are preferably gas phase fluidised bed polymerisation conditions.
Methods for operating gas phase fluidised bed processes for making polyethylene and ethylene copolymers are well known in the art. The process can be operated, for example, in a vertical cylindrical reactor equipped with a perforated distribution plate to support the bed and to distribute the incoming fluidising gas stream through the bed. The fluidising gas circulating through the bed serves to remove the heat of polymerisation from the bed and to supply monomer for polymerisation in the bed. Thus the fluidising gas generally comprises the monomer(s) normally together with some inert gas (e.g. nitrogen) and optionally with hydrogen as molecular weight modifier. The hot fluidising gas emerging from the top of the bed is led optionally through a velocity reduction zone (this can be a cylindrical portion of the reactor having a wider diameter) and, if desired, a cyclone and or filters to disentrain fine solid particles from the gas stream. The hot gas is then led to a heat exchanger to remove at least part of the heat of polymerisation. Catalyst is preferably fed continuously or at regular intervals to the bed. At start up of the process, the bed comprises fluidisable polymer which is preferably similar to the target polymer. Polymer is produced continuously within the bed by the polymerisation of the monomer(s). Preferably means are provided to discharge polymer from the bed continuously or at regular intervals to maintain the fluidised bed at the desired height. The process is generally operated at relatively low pressure, for example, at 10 to 50 bars, and at temperatures for example, between 50 and 120xc2x0 C. The temperature of the bed is maintained below the sintering temperature of the fluidised polymer to avoid problems of agglomeration.
In the gas phase fluidised bed process for polymerisation of olefins the heat evolved by the exothermic polymerisation reaction is normally removed from the polymerisation zone (ie, the fluidised bed) by means of the fluidising gas stream as described above. The hot reactor gas emerging from the top of the bed is led through one or more heat exchangers wherein the gas is cooled. The cooled reactor gas, together with any make-up gas, is then recycled to the base of the bed. In the gas phase fluidised bed polymerisation process of the present invention it is desirable to provide additional cooling of the bed (and thereby improve the space time yield of the process) by feeding a volatile liquid to the bed under conditions such that the liquid evaporates in the bed thereby absorbing additional heat of polymerisation from the bed by the xe2x80x9clatent heat of evaporationxe2x80x9d effect. When the hot recycle gas from the bed enters the heat exchanger, the volatile liquid can condense out. In one embodiment of the present invention the volatile liquid is separated from the recycle gas and reintroduced separately into the bed. Thus, for example, the volatile liquid can be separated and sprayed into the bed. In another embodiment of the present invention the volatile liquid is recycled to the bed with the recycle gas. Thus the volatile liquid can be condensed from the fluidising gas stream emerging from the reactor and can be recycled to the bed with recycle gas, or can be separated from the recycle gas and sprayed back into the bed.
The method of condensing liquid in the recycle gas stream and returning the mixture of gas and entrained liquid to the bed is described in EP-A-0089691 and EP-A-0241947. It is preferred to reintroduce the condensed liquid into the bed separate from the recycle gas using the process described in our U.S. Pat. No. 5,541,270, the teaching of which is hereby incorporated into this specification.
When using the catalysts of the present invention under gas phase polymerisation conditions, the catalyst, or one or more of the components employed to form the catalyst can, for example, be introduced into the polymerisation reaction zone in liquid form, for example, as a solution in an inert liquid diluent. Thus, for example, the transition metal component, or the activator component, or both of these components can be dissolved or slurried in a liquid diluent and fed to the polymerisation zone. Under these circumstances it is preferred the liquid containing the component(s) is sprayed as fine droplets into the polymerisation zone. The droplet diameter is preferably within the range 1 to 1000 microns. EP-A-0593083, the teaching of which is hereby incorporated into this specification, discloses a process for introducing a polymerisation catalyst into a gas phase polymerisation. The methods disclosed in EP-A-0593083 can be suitably employed in the polymerisation process of the present invention if desired.
In use, the polymers of the invention are conventionally compounded into pellets. Additionally or alternatively, additives may be incorporated into the polymers, such as antioxidants or neutralisers. In addition to being blown into films, the polymers are particularly suitable for making a variety of moulded or extruded articles. Thus the invention also includes within its scope a polymer as defined above in the form of pellets or a film or a moulded or extruded article. Such articles include pipes, and containers such as bottles or drums.